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Caught in the act: Hercules X-1 spins up while INTEGRAL is watching

17 June 2008

In spite of being one of the longest known and most studied X-ray sources, Hercules X-1 continues to intrigue astronomers. A recent report describing observations conducted with INTEGRAL confirms the picture of a complex emission pattern from this source, and for the first time shows the pulsar accelerating.

This is reported by Dmitry Klochkov and colleagues in an article published in the May 2008 issue of Astronomy & Astrophysics.

Artist's impression of the HZ Her/Her X-1 binary system

Hercules X-1 is a prototypical binary system formed by a pulsar (a neutron star) and a low mass companion star. In these systems mass flows from the companion onto the neutron star, forming an accretion disk. The interplay of the emission from the pulsar, the accretion disk, the orbital motion of the two objects around the common center of gravity, and the angle from which the system is viewed from Earth, sets the scene for possibly complex emission patterns, modulated with one or more of the characteristic periodicities of the system.

In this respect, Hercules X-1 is quite remarkable, displaying no less than four distinct periodicities: the pulsed signal from the pulsar (with a period of 1.24 s), its modulation due to the binary orbital motion (1.7 day), the 35 day on-off cycle due to the precession of the accretion disk, and the 1.65 day period of pre-eclipse dips.

The INTEGRAL observations, reported by Klochkov and colleagues, cover the period from 22 July to 3 August 2005. Data from the JEM-X, ISGRI, and SPI instruments were used, delivering data over almost two decades in energy, from 3 to 100 keV, beyond which energy the source becomes too weak to be detected.

The X-ray light curve of Her X-1 obtained with ISGRI (top) and JEM-X (bottom)

The photon collection power of INTEGRAL is such that the authors had enough statistics to slice the data both in time and in energy. Their analysis shows that the pulse profile of the pulsar changes with energy and time. This behavior has long been known, and a number of theoretical models have been developed: the INTEGRAL data are in agreement with the interpretation that some of the variability is caused by lumps of matter that move across the line of sight.

Variability in the rotation period of the pulsar is common in low mass X-ray binaries. In the case of Hercules X-1 period variations had been measured during distinct on phases of the 35 day cycle. Now, thanks to the high number of photons collected by INTEGRAL, the authors have been able to measure the pulsar spin rate at different times during the on state of the same 35 day cycle. The measurements are not compatible with a constant spin period, and clearly indicate that the pulsar is accelerating at a rate that is a few times the averaged spin up rate that has been previously reported. It is consistent with the typical difference between the values of pulse period between consecutive 35 day cycles.

This long INTEGRAL observation of Hercules X-1 provided an opportunity to probe deeper into the complex behaviour of this unusual accreting pulsar and to compare a number of models which have been proposed to explain the observed emission.